This invention relates to a rotary valve and to a method of forming a sleeve for such a valve and is primarily concerned with rotary valves of the type utilised for controlling fluid (usually hydraulic) to power assistance means of a steering gear. Rotary valves for power assisted steering gears are well known in the automotive industry and conventionally comprise a rotor mounted in the bore of a sleeve so that during a steering manoeuvre the rotor and sleeve are rotationally displaced relative to each other and in so doing adjust control ports in the valve to direct fluid to and from power assistance means as appropriate to assist the steering manoeuvre. The power assistance means will usually be in the form of a double acting piston and cylinder device incorporated in the steering gear. Examples of rotary valves of the type aforementioned are to be found in U.K. Patent Specifications Nos. 391,775, 476,590 and 1,356,172, U.S. Pat. Nos. 1,947,973, 2,328,312, 1,657,412 and 1,773,794. In each of these examples fluid flow through the valve is achieved by providing distributing zones on the rotor which are displaced during the aforementioned relative rotation to determine fluid flow between ports in the valves, such ports being connected to fluid pressure supply, the power assistance means and exhaust/reservoir (low pressure return). Since the early 1930s the most favoured sleeve structure has comprised a peripherally spaced array of axially extending, blind ended, recesses in the bore of the sleeve. Whilst this popular design is efficient and reliable in operation, it suffers from two serious disadvantages. Firstly, the design does not lend itself to economic manufacture on a mass production basis where either several assembly stages are required for producing the sleeve as a two or three part component as envisaged by the disclosure in Specifications: U.K. Pat. No. 476,590 and U.S. Pat. No. 1,947,973, or expensive special purpose machinery is necessary for manufacturing the sleeve as a one piece component as envisaged by the disclosure in U.S. Pat. No. 2,328,312 and U.K. Pat. No. 1,356,172. Secondly, the provision of the axially extending blind ended recesses calls for a relatively large sleeve and a correspondingly large housing within which the sleeve and rotor are mounted. This is incompatible with the desirability of providing a small, compact, steering gear as called for in modern vehicles. It is therefore an object of the present invention to provide a rotary valve for a power assisted steering gear and also a method of manufacturing the sleeve for such a valve which will lend themselves to the economic manufacture of a compact valve assembly without loss of efficiency or reliability when compared with conventional forms of rotary valves for power assisted steering.